The present invention provides a very simple configuration for stable initialization against the transient conditions of the source voltage.
The normal operation of the combustion control device is as follows. When the source voltage is supplied to the combustion control circuits, at first the blower is operated to prepurge the combustion chamber, drawing off the gas which remains in the combustion chamber, and after a predetermined time interval, the fuel supply device and the igniter are actuated, so that the igniting action is accomplished.
In conventional combustion control circuits, in the performance of these operations in proper sequence at the time of first application of power, the state of each flip-flop in the integrated circuits is randomly determined against a transient condition of the source voltage supplied to the semiconductor integrated circuits. In other words, the operation start signal for an operation switch or the like is provisionally reset after the source voltage is supplied for the purpose of initialization of the circuits.
A typical system will be described with reference to FIG. 1. The description below concerns a configuration for combustion control of a combustor.
Semiconductor integrated circuits for combustion control generally comprise a logic gate section 2 including a timer for taking a timing of combustion control and a logic circuit for generating various output signals in response to a signal from the timer and an input signal, an input interface section 1 for applying to the logic gate section 2 such signals as those from an operation switch, a flame detection signal and a fault alarm input (not shown) required for combustion control, and an output interface section 3 for amplifying the signal from the logic gate section 2 and driving the load such as a combustion blower or a solenoid valve (not shown). The input interface section 1, the logic gate section 2 and the output interface section 3 are connected to a common power supply Vcc as a voltage source. The logic gate section 2 typically comprises integrated injection logic (IIL); whereas, the input interface section 1 and the output interface section 3 comprise bipolar transistors.
When the source voltage Vcc is applied to the integrated circuits, a voltage change occurs with time as shown in the waveform of FIG. 2. Although the rise time T is actually very short (on the order of milliseconds) and apparently represents a sharp rise, the electronic circuit responds to this transient condition. The circuit, which is not actuated by a low voltage Vcc, begins to be actuated from a certain potential depending on the circuit configuration. The parts having the same circuit configuration are actuated at the same time. Memory elements such as flip-flops, therefore, are settled in a given condition depending on a slight difference in charges or voltage of the transistors making up the circuit. This condition is not always representative of the same mode and therefore the operation of these circuits is very unstable.
As the logic condition of each of the circuits is randomly determined when each of the circuits is activated after the transient condition of the source voltage, the output interface section may be undesirably operated before the initialization of the circuits is completed. This results in a chattering of the output signal from the control circuits when the source voltage is supplied. In a combustion control circuit, at the time of application of the source voltage, namely at the initial state of the circuits, all output signals of the circuits must reflect an "off state" condition; otherwise, malfunctions may occur in the circuit. For example, if the igniter is momentarily operated by the chattering of the output signal, in the case where gas remains in the combustion chamber, an explosuion may occur. Also, the combustion control circuit generally includes an alarm device which is operated in case of a malfunction to provide an alarm and stop the operation of the blower and the fuel supply device. Under these conditions, the blower and fuel supply device are controlled so that they cannot be undesirably restarted during the operation of the alarm device. Thus, where the alarm device is actuated due to a malfunction during initialization of the combustion control circuit, the entire system is effected.
It is thus necessary to apply a signal for resetting the whole of the logic circuit 2 through the input interface 1 after the source voltage is established, thereby complicating the circuit configuration.